The present invention relates to an optical transducer, which may be mentioned by way of example in relation to the shaft of a motor which provides for selection of the characters of a character-carrying element of an office typewriter or which produces movement of the character-carrying element with respect to a printing line. In this type of transducer, the movable member unambiguously determines the relative position as between the shutter member and the assembly formed by the light source and the photodetector, and causes illumination of the photodetector, which varies in dependence on the movement of the movable member. In response to illumination thereof, the photodetector in turn produces a signal which indicates the position of the movable member. In order to have sufficiently accurate information, the output signal must have a fairly high level and thus dimensions of the photodetector cannot be too small. Moreover any variations in response by the photodetector are detected as variations in the position of the movable member, so that the accuracy of the transducer directly depends on the stability of the photodetector and the illuminating means in regard to external disturbances or variation in time in respect of the characteristics of the various electrical-optical components.
A position transducer is known which comprises a shutter member in the form of a disc which has apertures or notches at its periphery and which is connected to a rotary member, a photodiode which illuminates a notched sector of the disc, and a mask provided with four regions which are provided with a series of apertures or notches corresponding to those of the disc. The apertures or notches of the four regions are out of phase relative to each other by a quarter of a pitch and selectively transmit the light from the apertures or notches of the disc to four photosensitive elements. The output signals of the photosensitive elements are used, in pairs in phase opposition, to produce two positional signals which in turn are out-of-phase by a quarter of a cycle. A part of the light emitted by the photodiode which passes by the periphery of the disc is detected by a further photodetector which provides pilot control for a circuit for stabilising the photodiode itself. Such a transducer has been found to be fairly accurate. However, during production and depending on the characteristics of the individual components, the transducer requires accurate and expensive individual calibration of the circuits associated therewith for adjusting the signals of the photodetectors to the level envisaged by the design. In the course of service moreover, the variations in distribution of the light flux or the variations in characteristics of any of the five sensitive elements which are disposed on various areas of the cone of illumination of the photodiode give rise to a positional error which cannot be corrected except by replacing the entire transducer.